1. Field of the Invention
The present invention relates to a method of making custom dental impression trays.
2. Prior Art
A denture, bridge, or cap or crown--or an accurate wax replica of such a dental prosthesis used in the manufacture of the finished prosthesis--usually is formed on a full size model of a patient's jaw ridge, that is, the ridge-formed parts of the gums, the adjacent teeth, if any, and the alveolar portions of the jaws, in the area to receive the prosthesis. A mold for the initially plastic material which hardens into such a model is obtained by taking an impression of the appropriate area of the patient's jaw ridge.
The appearance and, particularly, the fit of the finished prosthesis depends to a large degree on the preciseness of the model on which it was formed which, in turn, depends on the preciseness of the impression taken of the patient's jaw ridge in forming the mold for the model. Even a small error in the impression can ultimately result in an unstable or gum-irritating denture or a bridge or crown which causes adjacent tissue to be irritated or to recede.
Typically the deformable impression material is held in a tray having an arcuate channel or depression of a width slightly or substantially greater than the width of the jaw ridge in the area to receive the prosthesis. The tray containing the deformable impression material is placed in the patient's mouth, then is pressed firmly so that it surrounds the jaw ridge, causing the impression material to conform to the shape of the gums and teeth, and, after the impression material has hardened in contact with the gums and teeth, is removed from the patient's mouth so that the hardened impression material can be used as the mold for the model.
Since some of the tissues of which an impression is required are quite soft, the impression material used must be even softer to prevent distortion of the tissues as the impression material is pressed into contact with such tissues. The softness of the tissues also makes it extremely important that the impression tray correspond closely to the size and shape of the area to receive the prosethesis. As noted by C. W. Ellinger et al. at page 110 of their book Synopsis of Complete Dentures (Lea & Febiger, 1975), "An impression tray is the most important part of an impression, regardless of technique." If at any location the overall width of the tray is too great, or the width of the tray channel is too narrow, the gum tissues are stretched or compressed by the tray leading to an inaccurate impression. As stated at page 111 of Synopsis of Complete Dentures,
Since all edentulous ridges vary, it would seem that the ideal tray is one that is specifically made for the patient. The borders of this tray can be adjusted so that they control the movable soft tissues around the impression with little distortion to these tissues. At the same time, space may be provided inside the tray consistent with the characteristics of the impression material being utilized. PA1 taking a preliminary impression as discussed above; PA1 casting the study model using the preliminary impression as the mold; PA1 using the study model to form a "record base" or "baseplate" of shellac of a desired thickness which, in effect, duplicates the prelimary impression in a harder, thinner material, and removing the baseplate from the study model; PA1 inverting the baseplate into a soft patty of dental stone and, after the dental stone has hardened, removing the baseplate from the hardened stone, thereby forming an approximate mold or "former" for the custom impression tray; PA1 coating the dental stone tray mold with a separating agent, such as petroleum jelly; PA1 mixing acrylic resin according to the manufacturer's instructions, allowing the initially substantially liquid resin to set to the "doughy" stage and distributing the doughy acrylic resin evenly in the mold; and PA1 removing the acrylic resin from the mold while it is still pliable, placing it over the built-up study model, manually adapting it to the study model and allowing the acrylic resin to harden to form the custom tray.
The word "custom" is used herein to describe such an impression tray made specifically for an individual patient.
Synopsis of Complete Dentures, particularly in Chapter 10, also discusses several alternative methods for forming a custom dental impression tray. In each instance a preliminary impression is taken by use of a "stock" tray which may be available in three or four standard sizes. If plastic modeling compound is used as the preliminary impression material, the hardened compound can be removed from the tray, trimmed to the desired thickness and height, and the concave impression "relieved" by scraping or cutting away the hardened compound at desired locations leaving room for the final impression material. In this method the preliminary impression itself is used as the custom tray, and the custom tray is used to obtain a final impression which serves as the mold for the "master" model on which the dental prosthesis is formed.
A problem with this method is that the trimmed, hardened modeling compound is somewhat brittle. It is difficult to apply the localized pressure required to obtain a good final impression without breaking the trimmed hardened tray. As a result, a more popular method for making a custom impression tray is to use the initial impression as a mold for a "study" model on which a custom tray of acrylic resin can be formed.
Unfortunately formation of the acrylic resin custom tray is cumbersome and time consuming. The method recommended in Synopsis of Complete Dentures at pages 130 to 131 includes:
building up desired locations of the study model by use of wax;
By building up the study model, the resulting custom dental impression tray has room for the final impression material.
Regardless of the method used for manufacturing the custom dental impression tray, such tray is used to obtain a final impression which serves as the mold for making the highly accurate master model on which the dental prosthesis is formed.